zed/crates/gpui/src/arena.rs

use std::{
    alloc,
    cell::Cell,
    ops::{Deref, DerefMut},
    ptr,
    rc::Rc,
};

struct ArenaElement {
    value: *mut u8,
    drop: unsafe fn(*mut u8),
}

impl Drop for ArenaElement {
    #[inline(always)]
    fn drop(&mut self) {
        unsafe {
            (self.drop)(self.value);
        }
    }
}

pub struct Arena {
    start: *mut u8,
    end: *mut u8,
    offset: *mut u8,
    elements: Vec<ArenaElement>,
    valid: Rc<Cell<bool>>,
}

impl Arena {
    pub fn new(size_in_bytes: usize) -> Self {
        unsafe {
            let layout = alloc::Layout::from_size_align(size_in_bytes, 1).unwrap();
            let start = alloc::alloc(layout);
            let end = start.add(size_in_bytes);
            Self {
                start,
                end,
                offset: start,
                elements: Vec::new(),
                valid: Rc::new(Cell::new(true)),
            }
        }
    }

    pub fn len(&self) -> usize {
        self.offset as usize - self.start as usize
    }

    pub fn capacity(&self) -> usize {
        self.end as usize - self.start as usize
    }

    pub fn clear(&mut self) {
        self.valid.set(false);
        self.valid = Rc::new(Cell::new(true));
        self.elements.clear();
        self.offset = self.start;
    }

    #[inline(always)]
    pub fn alloc<T>(&mut self, f: impl FnOnce() -> T) -> ArenaBox<T> {
        #[inline(always)]
        unsafe fn inner_writer<T, F>(ptr: *mut T, f: F)
        where
            F: FnOnce() -> T,
        {
            ptr::write(ptr, f());
        }

        unsafe fn drop<T>(ptr: *mut u8) {
            std::ptr::drop_in_place(ptr.cast::<T>());
        }

        unsafe {
            let layout = alloc::Layout::new::<T>();
            let offset = self.offset.add(self.offset.align_offset(layout.align()));
            let next_offset = offset.add(layout.size());
            assert!(next_offset <= self.end, "not enough space in Arena");

            let result = ArenaBox {
                ptr: offset.cast(),
                valid: self.valid.clone(),
            };

            inner_writer(result.ptr, f);
            self.elements.push(ArenaElement {
                value: offset,
                drop: drop::<T>,
            });
            self.offset = next_offset;

            result
        }
    }
}

impl Drop for Arena {
    fn drop(&mut self) {
        self.clear();
    }
}

pub struct ArenaBox<T: ?Sized> {
    ptr: *mut T,
    valid: Rc<Cell<bool>>,
}

impl<T: ?Sized> ArenaBox<T> {
    #[inline(always)]
    pub fn map<U: ?Sized>(mut self, f: impl FnOnce(&mut T) -> &mut U) -> ArenaBox<U> {
        ArenaBox {
            ptr: f(&mut self),
            valid: self.valid,
        }
    }

    fn validate(&self) {
        assert!(
            self.valid.get(),
            "attempted to dereference an ArenaRef after its Arena was cleared"
        );
    }
}

impl<T: ?Sized> Deref for ArenaBox<T> {
    type Target = T;

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        self.validate();
        unsafe { &*self.ptr }
    }
}

impl<T: ?Sized> DerefMut for ArenaBox<T> {
    #[inline(always)]
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.validate();
        unsafe { &mut *self.ptr }
    }
}

pub struct ArenaRef<T: ?Sized>(ArenaBox<T>);

impl<T: ?Sized> From<ArenaBox<T>> for ArenaRef<T> {
    fn from(value: ArenaBox<T>) -> Self {
        ArenaRef(value)
    }
}

impl<T: ?Sized> Clone for ArenaRef<T> {
    fn clone(&self) -> Self {
        Self(ArenaBox {
            ptr: self.0.ptr,
            valid: self.0.valid.clone(),
        })
    }
}

impl<T: ?Sized> Deref for ArenaRef<T> {
    type Target = T;

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        self.0.deref()
    }
}

#[cfg(test)]
mod tests {
    use std::{cell::Cell, rc::Rc};

    use super::*;

    #[test]
    fn test_arena() {
        let mut arena = Arena::new(1024);
        let a = arena.alloc(|| 1u64);
        let b = arena.alloc(|| 2u32);
        let c = arena.alloc(|| 3u16);
        let d = arena.alloc(|| 4u8);
        assert_eq!(*a, 1);
        assert_eq!(*b, 2);
        assert_eq!(*c, 3);
        assert_eq!(*d, 4);

        arena.clear();
        let a = arena.alloc(|| 5u64);
        let b = arena.alloc(|| 6u32);
        let c = arena.alloc(|| 7u16);
        let d = arena.alloc(|| 8u8);
        assert_eq!(*a, 5);
        assert_eq!(*b, 6);
        assert_eq!(*c, 7);
        assert_eq!(*d, 8);

        // Ensure drop gets called.
        let dropped = Rc::new(Cell::new(false));
        struct DropGuard(Rc<Cell<bool>>);
        impl Drop for DropGuard {
            fn drop(&mut self) {
                self.0.set(true);
            }
        }
        arena.alloc(|| DropGuard(dropped.clone()));
        arena.clear();
        assert!(dropped.get());
    }

    #[test]
    #[should_panic(expected = "not enough space in Arena")]
    fn test_arena_overflow() {
        let mut arena = Arena::new(16);
        arena.alloc(|| 1u64);
        arena.alloc(|| 2u64);
        // This should panic.
        arena.alloc(|| 3u64);
    }

    #[test]
    fn test_arena_alignment() {
        let mut arena = Arena::new(256);
        let x1 = arena.alloc(|| 1u8);
        let x2 = arena.alloc(|| 2u16);
        let x3 = arena.alloc(|| 3u32);
        let x4 = arena.alloc(|| 4u64);
        let x5 = arena.alloc(|| 5u64);

        assert_eq!(*x1, 1);
        assert_eq!(*x2, 2);
        assert_eq!(*x3, 3);
        assert_eq!(*x4, 4);
        assert_eq!(*x5, 5);

        assert_eq!(x1.ptr.align_offset(std::mem::align_of_val(&*x1)), 0);
        assert_eq!(x2.ptr.align_offset(std::mem::align_of_val(&*x2)), 0);
    }

    #[test]
    #[should_panic(expected = "attempted to dereference an ArenaRef after its Arena was cleared")]
    fn test_arena_use_after_clear() {
        let mut arena = Arena::new(16);
        let value = arena.alloc(|| 1u64);

        arena.clear();
        let _read_value = *value;
    }
}